Method and apparatus for producing any desired M-phase, nth order electrical system in a converter-fed device

ABSTRACT

A method and apparatus for producing any desired m-phase, n-th order electrical system in a converter-fed device, in which an n-th order voltage system is formed in such a manner that a determined stationary voltage vector, which is related to the n-th order electrical system, is transformed from a reference system rotating at n-times the fundamental frequency into a fixed reference system, and up to m voltage pilot control signals are produced from these rotating voltage vectors. Using this method, any desired n-th order electrical system can be impressed on, or suppressed in, phase currents of a converter-fed device.

FIELD OF THE INVENTION

The present invention relates to a method for producing any desiredm-phase, n-th order electrical system in a converter-fed device, and toan apparatus for carrying out this method.

BACKGROUND INFORMATION

When converters are used to feed electrical systems, for examplethree-phase machines or three-phase mains systems, the current alsocontains, in addition to the desired first order positive phase-sequencesystem, n-th order systems which are undesirable, for example because ofthe additional losses that they cause.

If one wishes to suppress n-th order electrical systems, then acorresponding voltage system must be produced for this purpose, which issupplied, as pilot control for manipulated variables, to a converter-fedsystem.

The article "Koordinatentransforationen fur MehrgroBen-Regel-systeme zurKompensation und Symmetrierung von Drehstrom-netzen" (CoordinateTransformations for Multi-Variable Control Systems for Power FactorCorrection Balancing of Three-Phase Mains System) by W. Meusl and H.Waldmann, printed in the German journal "Siemens Forsch.--undEntwickl.--Ber.", (Siemens Research and Development Reports) Volume 6,1977, No. 1, pages 3 to 12, describes a control system, also called amulti-variable control sytem, for a solid-state power factor correctorwhich is used in a three-phase arc furnace. This control system alwaysdrives thyristors of the solid-state power factor corrector such thatthe reactive current load on the mains system is as low as possible, asconstant as possible and such that the load on the mains system overallis as balanced as possible. In the representation using symmetricalcomponents, this task is defined as follows:

The reactive element of the positive phase-sequence system of the mainscurrent should be as small and constant as possible. At the same time,the negative phase-sequence system should be as small as possible,overall.

Since there is no neutral connection in this arrangement, the zerophase-sequence system is always equal to zero.

The article summarizes the most important relationships between a numberof component systems, and illustrates these relationships in matrices inTable 1. When phase currents ((R,S,T) components) are converted intobalanced components ((0, 1, 2) components), a (0, α, β) system can alsobe used, as shown in Table 1. Implementation of direct conversionrequires greater equipment complexity.

In this multi-variable control system for a solid-state power factorcorrector, analysis of the furnace currents produces the n-phase andreactive current components of the positive phase-sequence system,characterized by index 1, and of the negative phase-sequence system,characterized by index 2. The n-phase current components of the positivephase-sequence system are not processed any further. The reactivecurrent components of the positive phase-sequence system and bothcomponents of the negative phase-sequence system are intended to becanceled out by the power factor correction system. These components areused as reference variables in this control system. The controlvariables are determined from the currents on the mains system side ofthe power factor correction system, using the same method as for thereference variables. Since this control system essentially has toprocess only reference variable changes, pilot control is used. Theoutputs of the positive phase-sequence system reactive current componentcontroller, of the. negative phase-sequence system in-phase currentcomponents controller and of the negative phase-sequence system reactivecurrent component controller are transformed into phase variables, whichare then converted into control signals for the thyristors of the powerfactor correction system. This control system is used to generatecontrol signals for each phase of the power factor correction system, asa result of which specific values are obtained for positivephase-sequence and negative phase-sequence systems on the mains side.

This control system influences only the components of the first orderpositive phase-sequence and negative phase-sequence system, and not n-thorder electrical systems.

In the case of a direct converter with an open circuit, harmonics canoccur despite sinusoidal control of the phase voltages if the backe.m.f. produced by the load, as a rule a motor, has a waveform which isnot sinusoidal. Special synchronous machines produce a back e.m.f. whichmay contain a high proportion of 3rd harmonics. In the case of thesalient pole machine which is used very frequently, it is possible, byappropriate design of the airgap, to achieve the production of a verygood sinusoidal back e.m.f. on no load, but, when loaded, fielddistortion occurs as a result of the pole axis being shifted withrespect to the axis of the rotating field, and causes correspondingharmonics of the back e.m.f. which is produced.

SUMMARY OF THE INVENTION

Production of any desired n-th order electrical system requires acorresponding n-th order voltage system. Such an n-th order voltagesystem can be produced for a stationary operating point using anempirically found voltage vector by transforming this empiricallydetermined stationary voltage vector from areference system rotating atn-times the fundamental frequency into a fixed reference system. Thisvoltage vector, which rotates at n-times the fundamental frequency, isconverted using coordinate transformation into voltage pilot controlsignals. The transformation variable for this transformation of thestationary voltage vector into a rotating voltage vector is determinedby means of an angle which changes in accordance with the fundamentalfrequency. The phase of this angle is freely variable. In the case of aconverter-fed three-phase machine, this may be the flux angle φ or therotor angle λ. The transformation variable, which varies at n-times thefrequency, is formed from this angle. The critical factor in theselection of the coordinate transformation is which n-th orderelectrical system (positive phase-sequence, negative phase-sequence orzero phase-sequence system) it is intended to produce. In addition, itis necessary to consider whether the specified method is intended to beimplemented in a cartesian coordinate system (analog implementation) orin a polar coordinate system (digital implementation). Furthermore, thenumber of phases of the system having a mutiple of three phases mustalso be considered. Depending on these boundary conditions, a 2/mconverter or a P/m converter, respectively, must be provided as thecoordinate converter for a positive phase-sequence or negativephase-sequence system, respectively, m being equal to the number ofphases of the multi-phase system. For a zero phase-sequence system, onlythe signal of the reference phase is required, and this is then fed toall the phases as a pilot control signal.

In a method according to the present invention, the stationary voltagevector is not predetermined empirically, but is generated from themeasured actual phase current values as a function of predeterminednominal value components. To this end, the actual phase current valuesare converted into components in a (0, α, β) system. These components ofthe (0, α, β) system each contain the information about all the n-thorder electrical systems. Predetermined nominal value components for thepositive phase-sequence, negative phase-sequence and zero phase-sequencesystem and the calculated components of the (0, α, β) are used to formcontrol difference current components in each case for the positivephase-sequence, negative phase-sequence and zero phase-sequence system,and these current components are vectorially rotated using cosine andsine angle functions at n-times the operating frequency. The result ofthis for the positive phase-sequence, negative phase-sequence and zerophase-sequence system is in each case two signals, which have a mutualin-phase element, depending on the phase angle. Thereafter, thesesignals are in each case integrated for each system, and which resultsin each case in a Cartesian component of a stationary voltage vector forthe positive phase-sequence, negative phase-sequence and zerophase-sequence system.

This method results in a stationary voltage vector of the positivephase-sequence, negative phase-sequence and zero phase-sequence system,which voltage vectors are automatically slaved to a changed operatingpoint in the converter-fed system. When the operating point changes, themagnitude and the phase of the n-th order electrical system also change.The components of the stationary voltage vector are changed inaccordance with this change.

In another embodiment of the method according to the present invention,which is intened to be applied only to an n-th order zero phase-sequenceelectrical system, the stationary voltage vector is not predeterminedempirically, but is generated from the measured actual phase currentvalues. The actual phase current values are added for this purpose, anda sum signal is obtained which contains the information about the sum ofall the zero phase-sequence electrical systems. This sum signal isvectorially rotated by means of cosine and sine angle functions atn-times the operating frequency, and two Cartesian components of the sumcurrent signal are obtained, which have a mutual in-phase element,depending on the phase angle. Thereafter, these components are eachintegrated and this results in each case in a cartesian component of thestationary voltage vector for the zero phase-sequence system. As aresult, a stationary voltage vector is obtained in a particularly simplemanner, and is slaved automatically to a changed operating point.

In a further embodiment of the method according to the presentinvention, a stationary voltage vector of a subsystem (positivephase-sequence, negative phase-sequence and zero phase-sequence system)is in each case formed by in each case delaying the signals, which areproduced by multiplication, of a subsystem, instead of by integratingthem. This results in a favorable solution for avoiding additionalproblems when limits of the converter-fed system come into play. Notonly limits resulting from the control limits of the converter beingreached, but also the control of active limits of the voltage vectormust be taken into account in this case. Delaying the signal from themultiplication means deliberately accepting a permanent control erroreven in the steady state, but is then no longer possible for anyintegrators to drift, which would corrupt the determination of thestationary voltage vector.

The magnitude of the permanent control error can be varied as a functionof a predetermined gain factor, so that this permanent control errorbecomes approximately zero.

The apparatus for carrying out the method according to the presentinvention includes vector rotators and coordinate converters, which aresufficiently known from field-oriented control. An implementation ofsuch components is achieved e.g., using a microprocessor, the additionalintegrators likewise being implemented in software when carrying out theadvantageous method.

In one embodiment of the apparatus according to the present invention,first order delay elements are used instead of the integrators, aP-regulator with a downstream limiter also being arranged in themagnitude channel of this apparatus. This refinement ensures that it isnot possible for any unlimited drifting to occur when the control limitof the m-phase converter is reached, as a result of which an optimumcontrol result is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an embodiment of the apparatus forcarrying out the method according to the present invention.

FIG. 2 shows a block diagram of an embodiment of the apparatus forcarrying out the method according to the present invention using polarcomponents.

FIG. 3 shows the actual phase voltage values plotted in a diagram as afunction of time t.

FIG. 4 shows an associated actual phase current values plotted in adiagram as a function of time t.

FIG. 5 showing the signal waveform at the outputs of the integratorsillustrated in FIG. 2 plotted in a diagram as a function of time t.

FIG. 6 showing a block diagram of yet another embodiment of theapparatus for carrying out the method according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a block diagram of a preferred embodiment of the apparatusfor carrying out the method according to the present invention for athree-phase system. This apparatus includes a device 2 for forming n-thorder control difference current components i1.sub.αe, i1.sub.βe,i2.sub.αe, i2.sub.βe and i0e, devices 4, 6 and 8 in each case forforming a stationary voltage vector u1₁, u1₂, u2₁,u2₂ and u0₁, u0₂ of apositive phase-sequence system i1m, a negative phase-sequence system i2mand a zero phase-sequence system i0m, and, for each subsystem i1m, i2m,i0m includes a transformation device 10, 12 and 14, downstream of eachof which a coordinate converter 16, 18 and 20 is connected. Each of thetwo outputs of a subsystem i1m, i2m and i0m of the device 2 is connectedto a device 4, 6 and 8. The transformation devices 10, 12 and 14 arelinked on the input side to the outputs of the device 4, 6 and 8. Theseries circuit formed by the devices 4, 6 and 8, respectively, thetransformation device 10, 12 or 14, and the coordinate converter 16, 18or 20, form a control channel 22, 24 or 26, respectively, for thepositive phase-sequence system i1m, the negative phase-sequence systemi2m or the zero phase-sequence system i0m, respectively, of thisapparatus.

The device 2 for forming n-th order control difference currentcomponents i1.sub.αe, i1.sub.βe 0r i2.sub.αe, i2.sub.βe or i0e, 0, ofthe positive phase-sequence system i1m, the negative phase-sequencesystem i2m or the zero phase-sequence system iom, respectively, includesa conversion device 28 and in each case two comparators 36, 38; 40, 42or 44, 46, respectively, for each system i1m, i2m or i0m, respectively.The conversion device 28 is supplied with the measured actual currentvalues i_(R), i_(S), i_(T), . . . , i_(m) of a converter-fed system. Theconversion device 28 is used to convert these actual current valuesi_(R),i_(S), i_(T), . . . , i_(m) into current components i.sub.α,i.sub.β and i₀. The current components i.sub.α, i.sub.β and i₀ thusproduce a new electrical system, which is obtained from the phasevariables i_(R),i_(S), i_(T), . . . i_(m) with the aid of realcoefficients. In the event of any unbalance, the current components(vectors) i.sub.α and i.sub.β are no longer of equal magnitude and areno longer at an angle of π/2 with respect to one another. The α and βcomponents of the (0, α, β) system produce, for example, thestator-oriented components. The stator-oriented components are in thiscase determined by the transformation formula as per formula (12) in thesaid article, which applies to the instantaneous values of the phasecurrents i_(R), i_(S) and i_(T) (and not only for vectors) as well, asis also expressed by the real coefficients. Formulae (11) and (12) inthe article described above thus apply both to the instantaneous valuesof the phase currents i_(R),i_(S) and i_(T) and to the vectors androtating vectors. For example, the matrix of formula 12 in the articledescribed by W. Meusl and H. Waldmann can be implemented in thisconversion device 28. These n-th order determined components i.sub.α andi.sub.β of the (0, α, β) system are in each case supplied to aninverting input of the comparators 36, 38 and 40, 42, respectively, thecomponent i0 being supplied to a comparator 44. Predetermined nominalvalue components i1*.sub.α, i1*.sub.β or i2*.sub.α, i2*.sub.β and iO*,respectively, are in each case present at the non-inverting inputs ofthese comparators 36, 38; 40, 42 and 44, respectively. The outputs ofthe comparators 36, 38; 40, 42 and 44, 46 are linked to the inputs ofthe device 4, 6 or 8, respectively, for forming a stationary voltagevector u1₁, u1₂ ; u2₁, u2₂ or u0₁, u0₂, respectively.

The devices 4, 6 and 8 for forming in each case one stationary voltagevector u1₁, u1₂, u2₁, u2₂ and u0₁, u0₂ are of approximately identicalconstruction, so that the construction will be explained in more detailwith reference to the device 4 in the positive phase-sequence systemi1m. The device 4 comprises a vector rotator 48 and two integrators 50and 52. The difference between the control channel 22 of the positivephase-sequence system and the control channel 24 of the negativephase-sequence system is that the vector rotators 48 and 56 in thepositive phase-sequence system have opposite mathematical signs for therotation angle to those of the vector rotators 48 and 56 in the negativephase-sequence system. The control difference current componentsi1.sub.αe and i1.sub.βe are present at the signal inputs of the vectorrotator 48. The transformation inputs of this vector rotator 48 areelectrically conductively linked to the outputs of the device 54 forforming a transformation variable cos nφ and sin nφ. The outputs of thisvector rotator 48 are each electrically conductively connected to anintegrator 50 or 52. The two components i1.sub.αe and i1.sub.βe containthe information relating to the sum of all the n-th order positivephase-sequence and negative phase-sequence electrical systems, withregard to the nominal value element under consideration. Themathematical sign of the rotation of the vector, characterized by itstwo current components i1.sub.αe and i1.sub.βe, gives the currentcomponents i1₁ and i1₂ at the outputs of the vector rotator 48, whosein-phase elements determine the difference for the n-th order positivephase-sequence system. If the in-phase elements are integrated in aclosed control loop, then a vector is obtained whose magnitude and anglecorrespond precisely to the stationary voltage vector u1₁, u1₂ to befound.

These determined stationary voltage vectors u1₁ and u1₂ allow an n-thorder voltage system to be produced in such a manner that thesestationary voltage vectors u1₁ and u1₂ are transformed using a vectorrotator 56 from a reference system which rotates at n-times the angularvelocity ω_(S), into a fixed reference system. A cartesian coordinatesystem 1/2 which rotates at n-times the fundamental frequency and hasany desired phase is provided as the rotating reference system. Thestator-oriented cartesian coordinate system α/β is provided as the fixedreference system. The transformation produces a voltage vector whichrotates at n-times the operating frequency and has the cartesiancomponents u1.sub.α and u1.sub.β. These components u1.sub.α and u1.sub.βare converted by means of a coordinate converter 16 into m voltage pilotcontrol signals UV1₁, . . . , m. In this case, m indicates the number ofphases of the system. In the case of a three-phase system illustratedhere, the coordinate converter 16 provided is a 2/3 converter at whoseoutputs three voltage pilot control signals UV1₁, UV1₂ and UV1₃ arepresent. Only the voltage pilot control system of the reference phase isrequired for pilot control of a zero phase-sequence system.

The transformation variables cos nφ and sin nφ vary at n-times thefundamental frequency ω_(S). These transformation variables cos nφ andsin nφ are formed using the device 54 for forming the transformationvariables cos nφ and sin nφ from an angle φ (flux angle) and a number n.The angle φ in this case varies at the instantaneous fundamentalfrequency. The flux angle φ or the rotor position angle λ can be used,as appropriate, for this purpose. In practice, both angles φ and λ areavailable in the control system of a converter-fed system. This angle φis multiplied by the cosine and sine angle functions using an anglefunction generator 60, and is further processed in the device 54. Thisangle function generator 60 may also be a component of the device 54.The angle function generator 60 is used to form, from the angle φ, atransformation variable cos φ and sin φ, which rotates at the operatingfrequency ω_(S) and is converted using the device 54 and the number ninto a transformation variable cos nφ and sin nφ, which rotates atn-times the fundamental frequency ω_(S). The device 54 may be providedfor example, by n vector rotators which are electrically connected inseries, the output signals of the angle function generator 60 beingpresent at the transformation inputs of each vector rotator.

If the components of the determined stationary voltage vector u1₁ andu1₂ are specified in polar coordinates (magnitude U, angle α), then theproduction of an n-th order voltage system for the positivephase-sequence system is considerably simplified. As shown in the blockdiagram in FIG. 2, which shows the control channel 26 of the zerophase-sequence system, only a multiplier 62 and an adder 64 are requiredin the control channel 22 of the positive phase-sequence system, insteadof the vector rotator 56, the device 54 and the generator 60. The angleφ or λ and the number n are present at the inputs of the multiplier 62.The output of the multiplier 62 is connected to one input of the adder64, the polar coordinate α of the stationary voltage vectors u1₁ and u1₂then being present at the second input of said adder 64. The output ofthis adder 64 is linked to one input of a coordinate converter 66. Thesecond polar component, namely the unchanged magnitude component U ofthe stationary voltage vectors u1₁ and u1₂, are present at the otherinput of this coordinate converter 66. The coordinate converter 66 usesthe polar components U and α+nφ to produce m voltage pilot controlsignals UV1₁, . . . , _(m). In the case of a three-phase system, a P/3converter would have to be provided as the coordinate converter 66, atwhose outputs the voltage pilot control signals UV1₁, UV1₂ and UV1₃ arepresent. Only the voltage pilot control signal of the reference phase isrequired for pilot control of a zero phase-sequence system. A P/1converter can thus be provided in the control channel 26 of the zerophase-sequence system, instead of the coordinate converter 66.

FIG. 2 shows the block diagram of the control channel 26 of the zerophase-sequence system i0_(m). If it is intended to influence only ann-th order zero phase-sequence system, then the control channels 22 and24 for the positive phase-sequence and negative phase-sequence systemare not required. The device 2 is likewise not required, since thecurrent component i0.sub.αe can also be determined in a simpler manner.The current component i0.sub.βe is equal to zero. The current componenti0.sub.αe is determined using an adder (which is not illustrated in moredetail) from the measured actual current values i_(R), i_(S) and i_(T)of a converter-fed system. This sum signal i0 contains the informationabout the sum of all the n-th order zero phase-sequence electricalsystems. In practice, a specific order zero phase-sequence system willdominate, the zero phase-sequence systems of other orders in contrastbeing negligible. As mentioned initially, third order harmonics areproduced in the phase currents in a direct converter with an opencircuit and with a sinusoidal drive. These third order harmonics form athird order zero phase-sequence system.

Subject to the precondition that the sum signal i0 of the zerophase-sequence electrical systems contains only a single n-th order zerophase-sequence system, this sum signal i0 has a sinusoidal waveform witha specific magnitude at n-times the operating-frequency and a phaseangle which is initially unknown. Linking the n-th order zerophase-sequence system to a sine and cosine function at the samefrequency (nφ) by means of the vector 48 produces two signals i0₁ andi0₂, which are at twice the frequency and have a mutual in-phaseelement, depending on the phase angle. The n-th order zerophase-sequence system is governed by the in-phase element in thesesignals i0₁ and i0₂. If the in-phase elements are integrated in a closedcontrol loop, then a vector is obtained whose magnitude and anglecorrespond precisely to the stationary voltage vectors u0₁ and u0₂ to befound.

In the steady state, the zero phase-sequence electrical system iscompletely eliminated, and the input signal of the two integrators 50and 52 is correspondingly zero. If the sum signal of the zerophase-sequence electrical systems includes a plurality of subsystems,the above arrangement can be constructed for each subsystem. Anon-suppressed zero phase-sequence system of any order results only init being possible for the instantaneous value at the integrated inputsnot to be zero as well, although this does not apply to the mean valueover a suitable time period. The integrators 50 and 52 can be designedin accordance with this boundary condition and the desired dynamicresponse.

The phase of the angle φ for the transformation may assume any value.Since the device 8 provides the output signal in cartesian componentsu0₁ and u0₂, and the circuit shown in FIG. 2 processes signals in polarcoordinates, a C/P converter 68 is provided at the interface. Inaddition, an angle function generator 60 is provided, which converts theangle nφ which is formed into a transformation variable cos nφ and sinnφ. Furthermore, a P/1 converter 66 is provided instead of the P/mcoordinate converter 20, since a pilot control signal UV0 is supplied toall the phases when an n-th order zero phase-sequence system issuppressed.

According to the block diagram in FIG. 1, the control difference currentcomponents i1.sub.αe and i1.sub.βe ; i2.sub.αe and i2.sub.βe, andi0_(e), respectively, in the control channel 22 of the positivephase-sequence system, in the control channel 24 of the negativephase-sequence system, and in the control channel 26 of the zerophase-sequence system, respectively, are produced on the input side ofthe vector rotator 48. The specified control difference currentcomponents are thus determined in a fixed reference system (α/β). If itis intended to generate these control difference current components in arotating reference system (1/2), then the comparators 36 and 38, 40 and42, and 44 must be connected downstream of in each case one output ofthe vector rotator 48 of the positive phase-sequence system, of thenegative phase-sequence system and of the zero phase-sequence system,respectively, the outputs in each case being linked to an invertinginput of the comparators 36 and 38, 40 and 42, and 44. The nominal valuecomponents are now provided as identical variables in this arrangement.

The functionality of the circuit arrangement described above has beenverified using a simulation (FIGS. 3 to 5). A zero phase-sequencevoltage system of order n=3 was installed in the supply to a synchronousmachine. This likewise results in a zero phase-sequence electricalsystem of order n=3. Since phase current regulators are present in thisspecial case of simulation, the converter voltages u_(A), u_(B) andu_(C) (FIG. 3) contain a zero phase-sequence system of order n=3 evenbefore the integrators 50 and 52 are enabled. This zero phase-sequencevoltage system is not preset correctly in terms of magnitude and phaseuntil the integrators 50 and 52 have been enabled at the time t1, sothat the system is a pure first order positive phase-sequence systemafter the correction process with respect to the current (FIG. 4). FIG.5 shows the signal waveforms u0₁ and u0₂ at the outputs of theintegrators 50 and 52 of the control channel 26 of the zerophase-sequence system.

FIG. 6 shows a particularly advantageous embodiment of the apparatus forcarrying out the method for suppression of an n-th order zerophase-sequence electrical system. This apparatus differs from theapparatus shown in FIG. 2 in that first order delay elements 70 and 72are provided instead of the integrators 50 and 52 in the device 8 forforming a stationary voltage vector u0₁ and u0₂. In addition, a Pregulator 76 with a downstream limiter 78 is arranged in a magnitudechannel 74.

In the arrangement shown in FIG. 2, the integrators 50 and 52 drift whenlimits come into effect. Not only limits on reaching the control limitsof a converter-fed system, but also the control of active limits of thevoltage vector must be taken into account in this case. In order tosolve this problem, first order delay elements 70 and 72 are providedinstead of the integrators 50 and 52. In consequence, a permanentcontrol error is deliberately accepted even in the steady state,although this can be influenced by the gain k of the P regulator 76 inthe magnitude channel 74. If the magnitude of the voltage vector kU issubsequent1y limited, this has no influence on the angle component α.This means that, if the magnitude of the voltage vector kU is activelylimited, the optimum angle value α+nφ is determined by the circuit, asbefore. An optimum control result is achieved in these given boundaryconditions by using the apparatus shown in FIG. 6. This furthermoreensures that it is impossible for any unlimited drifting to occur whenthe control limits of a converter-fed system are reached.

This method and this apparatus for carrying out the method allow thecontrol of a direct converter with an open circuit to be modified insuch a manner that a direct converter with an open circuit and with asinusoidal drive can be controlled in this way without the phasecurrentscontaining any third order harmonics, which contribute nothingto the useful power in the load but lead to an unnecessary additionalload on the converter. The direct converter with an open circuit and inconjunction with the method according to the present invention thusachieves a useful power output which is comparable with those of directconverters with a concatenated circuit, there being no need to increasethe supply voltage provided by the transformer, nor to use inductorcoils.

What is claimed is:
 1. A method for producing an m-phase/n-th orderelectrical system in a converter-fed device, the converter-fed deviceincluding an m-phase positive phase-sequence system, an m-phase negativephase-sequence system and a zero phase-sequence system, the methodcomprising the steps of:for each of the phase-sequence systems, formingan n-th order voltage system; determining a stationary voltage vectorcorresponding to an n-th order current component of a respectivephase-sequence system; transforming the determined stationary voltagevector from a reference system into a fixed reference system, thereference system rotating at n-times a fundamental frequency; and foreach of the phase-sequence systems, converting the determined stationaryvoltage vector into m voltage pilot control signals for controllingmanipulated variables.
 2. The method according to claim 1, wherein eachstationary voltage vector of the respective phase-sequence system isdetermined as a function of a respective m determined actual currentvalue and a respective predetermined nominal value component.
 3. Themethod according to claim 2, wherein the step of determining thestationary voltage vector includes:forming the n-th order currentcomponent using a respective predetermined reference value component anda respective current component, the respective current component beingdetermined as a function of a respective measured phase current of afixed reference system; vectorially rotating the formed n-th ordercontrol difference component using a sine function and a cosine functionat n-times the operating frequency; and delaying the rotated n-th ordercontrol difference component.
 4. The method according to claim 3,wherein each stationary voltage vector of the respective phase-sequencesystem includes vector components that are varied as a function of apredetermined gain factor.
 5. The method according to claim 1, whereinthe step of determining the stationary voltage vector includes:formingthe n-th order current component using a respective predeterminedreference value component and a respective current component, therespective current component being determined as a function of arespective measured phase current of a fixed reference system;vectorially rotating the formed n-th order control difference componentusing a sine function and a cosine function at n-times the operatingfrequency; and integrating the rotated n-th order control differencecomponent.
 6. An apparatus for producing an m-phase/n-th orderelectrical system, comprising:a coordinate converter; an m-phasepositive phase-sequence system forming a first n-th order voltage systemand including a first transformation device being coupled to an outputof the coordinate converter; an m-phase negative phase-sequence systemforming a second n-th order voltage system and including a secondtransformation device being coupled to the output of the coordinateconverter; a zero phase-sequence system forming a third n-th ordervoltage system and including a third transformation device being coupledto the output of the coordinate converter, wherein each of the first,second and third n-th order voltage systems includes a stationaryvoltage vector corresponding to an n-th order current component of arespective one of the phase-sequence systems, the stationary voltagevector being transformed from a reference system into a fixed referencesystem, the reference system rotating at n-times a fundamentalfrequency, wherein each of the n-th order voltage systems are convertedinto m voltage pilot control signals, the pilot control signals beingused for a pilot control of manipulated variables.
 7. The apparatusaccording to claim 6,wherein each of the transformation devices includesan angle function generator having first and second angle outputs, avector rotator having transformation inputs, and a forming device forforming a transformation variable, wherein the first angle outputreceives a predetermined value signal, and the second angle outputreceives signals from forming inputs of the forming device, wherein thetransformation inputs are coupled to forming outputs of the formingdevice, the transformation inputs providing a component of thestationary voltage vector, and wherein the angle function generatorincludes generator inputs having a corresponding angle operating at anyfrequency and at any phase.
 8. The apparatus according to claim 6,further comprising:a conversion device having conversion inputs andoutputs; and a determining device for determining the stationary voltagevector, the determining device including:determining inputs coupled tothe conversion outputs, two comparators having non-inverting inputsreceiving predetermined nominal value components, two integrators, and avector rotator having transformation inputs and outputs, thetransformation inputs being coupled to the determining device forforming the transformation variable, the transformation outputs beingcoupled to at least one of the two integrators via a respective one ofthe two comparators, wherein each of the transformation devices isconnected to the determining device, and wherein the conversion inputsreceiving measured current values.
 9. The apparatus according to claim6, further comprising:a first forming device for forming the n-th ordercurrent component and including first inputs; and at least one secondforming device for determining a stationary voltage vector, the secondforming device including second inputs and outputs, the second inputsreceiving actual inputs m measured current values predetermined nominalvalue components and being couple to the first outputs, wherein each ofthe transformation devices is connected to one of the at least onesecond forming device.
 10. The apparatus according to claim 9, whereineach of the at least one second forming device includes:two integrators,and a vector rotator having transformation inputs and outputs, thetransformation inputs being coupled to the at least one second formingdevice for forming the transformation variable, the transformationoutputs being coupled to the two integrators.
 11. The apparatusaccording to claim 9, wherein the first device includes:a conversiondevice downstream connected to actual value inputs of the first deviceand including converting outputs, and a plurality of comparators havinginverting and non-inverting inputs, the inverting inputs being coupledto one of the converting outputs, the non-inverting inputs being coupledto a nominal value input of the first device.
 12. The apparatusaccording to claim 9, wherein each of the at least one second formingdevice includes:a first coordinate converter, first order delay elementshaving delay outputs coupled to the first coordinate converter, and avector rotator having transformation inputs and outputs, thetransformation inputs being coupled to the at least one second formingdevice for forming the transformation variable, the transformationoutputs being coupled to the first order delay elements.
 13. Theapparatus according to claim 12, wherein each of the at least one secondforming device further includes:a second coordinate converter, and acomponent channel coupling the first coordinate converter with thesecond coordinate converter, the second coordinate converter including aP regulator and a downstream limiter.
 14. A method for producing anm-phase/n-th order electrical system in a converter-fed device, theconverter-fed device including an m-phase positive phase-sequencesystem, an m-phase negative phase-sequence system and a zerophase-sequence system, the method comprising the steps of:for each ofthe phase-sequence systems, forming an n-th order voltage system;determining a stationary voltage vector corresponding to an n-th ordercurrent component of a respective phase-sequence system; transformingthe determined stationary voltage vector from a reference system into afixed reference system, the reference system rotating at n-times afundamental frequency; for each of the phase-sequence systems,converting the determined stationary voltage vector into m voltage pilotcontrol signals for controlling manipulated variables; and providing them voltage pilot control signals to a direct converter having an opencircuit.